The present invention relates to oxygen sensors for measuring the oxygen content of molten metals, particularly molten iron, and more particularly the invention relates to improvements in the electrical contact structure between the solid electrolyte galvanic cell and the powder reference material in such oxygen sensors.
While research works and development of devices for measuring the oxygen content of molten iron have been carried out by many different companies all over the world, all of these devices are based on the processes which are basically similar in nature. Particularly, the solid electrolyte galvanic cells (e.g., ZrO.sub.2 -Ca.sub.2, ZrO.sub.2 -Y.sub.2 O.sub.3 or ZrO.sub.2 -MgO) which are used as ionic conductors in such devices are for the most part formed into a simple cylindrical shape, and the contact structure between the solid electrolyte galvanic cell and a powder reference material (such as, mixed powder of Cr and Cr.sub.2 O.sub.3 or mixed powder of Mo and MoO.sub.2) is in the form of a simple surface-to-surface contact.
The typical shape and mounting of the solid electrolyte galvanic cells in the known oxygen sensors are shown in FIG. 1. In the Figure, a cylindrical solid electrolyte galvanic cell 1 is mounted inside a quartz tube 3 by fusion or cementing (the portion indicated at numeral 2), and the solid electrolyte galvanic cell 1 makes, within the quartz tube 3, a surface-to-surface contact with a powder reference material 4 contained in the quartz tube 3 below the solid electrolyte galvanic cell 1. If the device in this condition is immersed into molten iron from the direction of an arrow 7, an electromotive force corresponding to the partial pressure of oxygen in the molten iron is produced across the solid electrolyte galvnic cell 1 or between the molten iron and the powder reference material 4 and delivered by way of electrode lead wires 5 and 6, and consequently the concentration of oxygen dissolved in the molten iron can be determined by measuring the thus delivered electromotive force.
When the oxygen sensor of the type shown in FIG. 1 is immersed into molten iron, an apparent contraction is caused in the powder reference material 4 upon occurrence of its sintering phenomenon due to high temperature, and the solid electrolyte galvanic cell 1 and the powder reference material 4 contacting each other in surface-to-surface relation are separated from each other, thus causing contact failure, causing electrically deflective conduction with the resulting increase in electric resistance, making it impossible to satisfactorily measure the generated electromotive force and thereby deteriorating the resulting emf curve which will be described later and causing the measurement to end in failure.
To overcome these deficiencies, the prior art devices of the type disclosed for example by the invention of U.S. Pat. No. 3,772,177 have been proposed. In the prior art device of this type, as shown in FIG. 2, that portion of an electrolyte galvanic cell 1 contacting a reference material 4 has the shape of a frustrum, and this frustrum 9 is embedded in and surrounded by the reference material 4. Also in this case, however, when the reference material 4 contracts, the reference material 4 slidingly contracts in the direction of the arrow, thus in most instances causing the electrolyte galvanic cell 1 and the reference material 4 to separate from each other and thereby causing the measurement to end in failure. The results of the actual tests made with this prior art device showed that the measured emf curves were for the most part unsatisfactory as shown in FIGS. 17, 18 and 19 of the accompanying drawings. Also the results of the tests made by modifying the shape of the frustrum 9, namely, by forming that portion of the electrolyte galvanic cell contacting the reference material into a conical shape entirely and embedding this conical portion in the reference material, showed about the same results as obtained with the frustrum and the rate of success in measurements was not improved considerably.